It depends on what size is available in what I want. Right now I have 6 of what I call "micro mini" slings. Which is 1/8 inch or less.
I have 5 A. seemani slings which are probably 3rd or 4th instar and an E. campestrautus that can't be much more than 4th instar.
I also have a really tiny A. minatrix that scares me silly. I check in on it a lot.
whatever size I can get for a reasonable price. If its a harder to get species i go for slings. Fast growers that I dont care about sex I go for slings. Cheaper ones that I really want a female of, I get sexed juvies+ Example:
my p ornata i got as a 1" sling, my g pulchra i got as a 1" sling, but my Cyripagopus sp "blue" i got as a sub-adult female.
I too, prefer to get them young- sling to juvie/subadult. That being said, I wouldn't hesitate to buy a more adult T of something slow growing that lives for a long time. Unfortunately, big, slow growing T's usually command a premium price, so the wallet tends to make some decisions for you! Let's see, one 6" Grammostola pulchra or 6 slings? hmmmm